Color image forming apparatus

ABSTRACT

A color copier which can be adopted to a printer operation for a peripheral imaging machine. The color copier forms plural color component toner layers on a photoreceptor layer by layer at each rotation of the photoreceptor. In the mode that the color copier is used as a printer, the copier makes two pages of each of the color component toner layers corresponding to an image at one rotation of a rotatable photoreceptor, when the size of the image is not more than one half the maximum size that can be formed by the copier.

BACKGROUND OF THE INVENTION

The present invention relates to a color image forming apparatus whereina toner image is formed on an image forming member through anelectrophotographic system and the toner image is transferred onto atransfer material for obtaining a color image.

Owing to advancement of technologies, color image forming apparatuseshave been developed by manufacturers recently. For example, JapanesePatent Publication Open to Public Inspection Nos. 100770/1985,149972/1986 and 75850/1985 (hereinafter referred to as Japanese PatentO.P.I. Publication) disclose methods for obtaining color copies from acolor document, wherein based on image data obtained by reading adocument by means of scanning by a scanner of an image reading unit,latent image formation and development thereof are conducted on aphotoreceptor drum for the frequency corresponding to the number ofcolors separated and transferred onto a transfer material to obtain acolor image by superposing toner images each having its own color.

Incidentally, in an office where a image forming apparatus is usuallyused, popular paper sizes each forming an image of one unit includeordinary paper sizes (JIS standards) of A-3, A-4, B-4 and B-5.Therefore, the circumferential length and the width of an image formingmember in an image forming apparatus such as a copying machine or thelike need to be in size covering at least one of the aforementionedpaper sizes. Namely, the circumferential length of the image formingmember must include, in addition to the length of the maximum papersize, additional length such as the one for the standard toner imageforming area for toner concentration detection and the one correspondingto a period of time necessary for a scanner of an image reading unit toreturn. The additional length necessary for the scanner to returnoccupies the greatest part of the total additional length.

However, the printing out speed in an image forming apparatus is thesame for all paper sizes. For example, the printing out speed for A-3size paper is exactly the same as that for A-4 size paper although thelength of A-4 size paper is 210 mm which is a half the size of A-3 sizepaper. The reason behind this is that only a portion of the imageforming member is used for image forming. For example, when printing outA-4 size paper under the condition that the maximum paper size is A-3,more than half of the image forming member is not used.

To be concrete, the image forming speed in the image forming apparatusis always the same regardless of the size of an image to be formed onthe image forming member, which has been wasteful. In other words, thecircumferential length of the image forming member is determined to copewith the maximum image size to be formed, and when the image size islarge, the entire surface of the image forming member is used for imageforming, while when the image size is small, only a small portion of theimage forming member is used for image forming, resulting in a wastefulportion of the image forming member.

Furthermore, if the recent trend of adding high functions to a colorimage forming apparatus is provided, in addition to its original copymode, with a print mode as a printer function for forming an image on atransfer material through the image forming system of the image formingapparatus using image data from an output device such as a computer or aword processor, it is not necessary to consider the width for a scannerto return when forming an image using image data from a peripheraloutput device.

The present invention has been achieved to solve the aforementionedproblems, and its object is to provide a color image forming apparatusforming an image of a smaller size through the process that is morecomplicated than the process for an image of a larger size, wherein thenumber of image frames formed in a single operation is increased, andthereby the wasteful portion on an image forming member is eliminatedand efficiency of operation is enhanced. Another object of the inventionis to provide a color image forming apparatus wherein thecircumferential length of an image forming member is minimized, thus,the apparatus may be caused, to be small in size and to be light inweight. A further object of the invention is to provide a color imageforming apparatus capable of forming images with an optimum frequency ofimage forming and with high efficiency, in all image forming modes.

SUMMARY OF THE INVENTION

The aforementioned objects of the invention are attained by a colorimage forming apparatus comprising a copy mode for controlling an imageforming unit wherein a toner image is formed when n image data readingscanning for one color, made by an image reading unit, corresponds to1-round or 2-round process of an image forming member depending on thesize of a color image to be formed, and a color image is formed when thetoner image forming mentioned above is repeated several times andthereby plural toner images, each having a different color, aresuperimposed, and comprising a print mode for controlling an imageforming process unit wherein a toner image is formed when 1-page or2-page image data inputted from a peripheral output device correspondsto 1-round process of an image forming member depending on the size of acolor image to be formed, and a color image is formed when the tonerimage forming, mentioned above, is repeated several times and therebyplural toner images each having a different color are superimposed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the main structure of a color copyingmachine equipped with a belt-shaped image forming member that is anexample of the invention, and

FIG. 2 is a control block diagram relating to a process control meansand an image processing means. FIG. 3 is a timing chart for eachprocessing unit in 1-round (page) process, and

FIG. 4 is a time chart for each processing unit in 2-round process.

FIG. 5 is a timing chart for each processing unit in 2-page process, and

FIG. 6 is a flow chart for selection of 1-page process or 2-pageprocess.

FIG. 7 is a drawing showing a color image forming apparatus of atransfer drum type that is another example of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Examples of the invention will be explained as follows, referring to thedrawings attached.

FIG. 1 shows the main structure of a color copying machine, that is acolor image forming apparatus equipped with a belt-shaped image formingmember, as an example of the invention. The color copying machine iscomposed roughly of image reading unit A and image forming unit B.

The image reading unit A will be explained first, referring to FIG. 1,which is a diagram showing the main structure of the color copyingmachine.

In the image reading unit A, a document placed on document table 11 isilluminated by halogen lamp 121 affixed to carriage 12 that slideshorizontally. Movable mirror unit 13 on which mirrors 131 and 132 areprovided also slides in a horizontal direction, and it guides a lightimage of the document to image reading portion 14 together with mirror122 affixed to the carriage 12 mentioned above.

The aforementioned carriage 12 and the movable mirror unit 13 mentionedabove are driven through a wire (not shown in the drawing) connected toa stepping motor, and move in the same direction at the speed of V andthe speed of 1/2 V, respectively.

The aforementioned image reading portion is provided with color CCDs 142at the position of lens 141 and the position for image formation that isbehind the lens 141. A document is subjected to color separation bymeans of the color CCD 142 to produce blue image data (B), green imagedata (G) and red image data (R) all of which are outputted to imageprocessing means 50 which will be explained later,

As shown in FIG. 2, image processing means 50 is composed of A/Dconverters 501B, 501G and 501R, complementary color converter 502, blackcomponent extraction unit 503 and masking unit 504.

The A/D converters 501B, 501G and 501R convert color image data composedof image data of blue (B), green (G) and red (R) inputted from colorCCDs 142 of the image reading unit A to image data of digital signalswith 256 steps of gradation, for example, and send them to thecomplementary color converter 502 which will be explained later.

The complementary color converter 502 complementary-color-converts thedigital signals of color image data converted by the A/D converter 501to obtain image data of yellow (Y), magenta (M) and cyan (C).

The black component extraction unit 503 extracts image data of black(Bk) from image data of Y, M and C obtained by the complementary colorconverter 502 through, for example, under color removal (UCR).

The masking unit 504 conducts color correction on image data of Y, M andC after they have been subjected to UCR.

Selector 510 selects image data of colors for latent image formationfrom image signals of Y, M, C and Bk processed by the image processingmeans 50.

Pattern generator 511 conducts pulse width modulation processing for Y,M, C and Bk signals which are color-corrected chromatic color componentsby comparing with a reference wave, and outputs thepulse-width-modulated image signals of Y, M, C and Bk in the sequence oftheir surfaces. The image data of the aforesaid image signals areoutputted to laser writing unit 20 in image writing unit B.

Further, image signals from peripheral output device 52 such as acomputer or a word processor are outputted to black component extractionunit 503 of image processing means 50. The image data, however, may alsobe outputted to laser writing unit 20 directly.

With regard to image signals after being subjected to complementarycolor conversion, black component extraction or color correction bymeans of image processing means 50, or image signals outputted fromperipheral output device 52, either one of image signals of yellow (Y)as the first color, image signals of magenta (M) as the second color,image signals of cyan (C) as the third color and image signals of black(Bk) as the fourth color, are outputted to laser writing unit 20 that isan exposure means when a latent image of each color is formed.

Next, image forming unit B will be explained as follows, referring toFIG. 1 which is a diagram, of primary structure of the color copyingmachine of the present invention.

In FIG. 1, photoreceptor belt 15 that is an image forming member is onewherein a photosensitive layer is provided on the surface of itsflexible belt through a method of evaporation or coating, and it isspread between driving roller 16 and driven roller 17. The drivingroller 16 rotates through a driving gear (not shown) that is engagedwith a gear provided on apparatus main body 10, and it causes thephotoreceptor belt 15 to rotate clockwise. Further, by means ofclearance-keeping member 18, the clearance between developer 21 and thephotoreceptor 15 is kept constant, enabling an excellent image to beformed stably. Although, photoreceptor belt 15 is used as an imageforming member in the present example, the invention is not limited onlyto this but is applicable also to a conventional image forming memberhaving thereon a photosensitive layer such as a photoreceptor drum.

There are provided charging means 19, exposure means 20, developingmeans of developers 21Y, 21M, 21C and 21Bk, transfer means 22 andcleaning means 23 around the photoreceptor belt 15.

The charging means 19 is provided for the purpose of uniformly chargingwith a predetermined polarity a photosensitive layer on the surface ofthe photoreceptor belt 15, and it may be represented by conventionalcharger 19 such as a corona charger or a scorotron charger, and thescorotron charger is preferably used for an OPC photoreceptor.

The exposure means is semiconductor laser writing unit 20 and it causesthe surface of the photoreceptor belt 15 charged by the charger 19 to beexposed so that an electrostatic latent image may be formed on thesurface.

The developing means comprises plural developers 21Y through 21Bk eachcontaining a differently colored developer, for example, a differentlycolored toner (developer) out of, for example, yellow, magenta, cyan andblack. Each of these developers 21Y through 21Bk has the function ofvisualizing an electrostatic latent image on the photoreceptor belt 15as a toner image through a non-contact developing method. Thisnon-contact developing method, unlike a contact developing method,neither damages a toner image formed in the previous cycle on thephotoreceptor belt 15 nor interrupts the movement of the photoreceptorbelt 15. Therefore, it may offer an excellent color image. Thedeveloping means is not limited only to the color development using fourcolors, as in the present example, but it is also applicable tomono-color, two-color or three-color development, wherein the developersin quantity corresponding to the number of toner colors to be used haveonly to be provided around the photoreceptor belt 15.

The transfer means transfers toner images formed on the photoreceptorbelt 15 onto a transfer material by means of transferring unit 22 suchas a transfer corona discharger. In place of the transferring unit 22, aconventional transferring member such as a transfer drum may also beused as a transfer means.

The cleaning means 23 is provided with cleaning blade 231, and isarranged so that it may be kept away from the surface of thephotoreceptor belt 15 during the image forming process and may furtherbe caused to be in pressure-contact with the surface of thephotoreceptor belt 15 for cleaning it only when cleaning the surfaceafter transferring toner images on the surface onto the transfermaterial.

Toner collecting box 24 is one wherein toners removed by the cleaningmeans 23 from the photoreceptor belt 15 are collected therein throughtoner collecting tube 241 by means of a waste toner screw.

Toner hopper 25 is one for storing toners to be supplied by a supplyscrew (not shown) to the aforementioned developers 21Y through 21Bk.

In the present example, processing units such as photoreceptor belt 15,charging unit 19, developers 21Y through 21Bk, cleaning means 23, tonercollecting box 24 and hopper 25, of which an image forming processportion of image forming unit B in the image forming apparatus mentionedabove is composed, are provided in process cartridge 26 in a body toform a unit which may be mounted detachably on the apparatus main body10. However, processing units to be provided in the process cartridge 26to form a unit are not limited to those mentioned above, and they arenaturally allowed to form a unit which is different from the presentexample.

The image forming apparatus 10 in the present example is a copyingmachine provided with a copy mode for obtaining a copy through theprocess of image-forming for data read from a document and a print modefor forming an image on a transfer material based on image data fromoutput devices such as a computer or a word processor through the imageforming unit of the present apparatus, thus, having also a printingfunction.

For the purpose of simplifying explanation of the present example, anexample wherein the same images are formed continuously on the imageforming apparatus 10 whose maximum paper size is A-3 will be stated asfollows. Incidentally, the circumferential length of the photoreceptorbelt 15 is assumed to be 420+α mm because the length of A-3 size is 420mm. (In this case, α=30-100 mm. This value represents a necessary lengthwhich does not reflect that carriage 12 returns to its initial positionwhen a document in A-3 size is copied.)

Copy Mode

First, a copy mode to conduct image forming for data read from adocument, namely, a process relating to a copy of color image formation,carried out by a color image forming apparatus having the aforementionedstructure, will be explained, referring to FIGS. 1-3.

In the arrangement of the present example, the maximum image size isA-3, the circumferential length of the photoreceptor belt 15 isdetermined to be the minimum necessary length to cover the A-3 size inorder to make the apparatus main body 10 small, and carriage 12 andmovable mirror unit 13 can not return to their initial positions withina period of time for the photoreceptor belt 15 to make one turn.Therefore, an explanation will be given, discriminating between copyingfor sizes other than A-3 size and copying for A-3 size. Incidentally, inthe following example, an explanation will be given for the case whereimages are formed on a transfer material of A-4 size and a transfermaterial of A-3 size.

Copy of A-4 Size

When copy signals are outputted from process controller 53, inputcontrolling unit 54 and image forming controller 55 control imagereading unit A and image forming unit B based on the copy signals.

The input controlling unit 54, when copy signals are inputted, drives animage reading optical unit composed of the carriage 12 and movablemirror unit 13 and reads a document on document table 11 as color imagedata of blue image data (B), green image data (G) and red image data (R)at image reading portion 14. Then, the read color image data aresubjected to complementary color conversion, black component extractionand color correction, carried out by the image processing means 50 sothat they may be image-processed as color image signals of Y, M, C andBk.

Then, selector 510, controlled by the process controller 53, selects andoutputs color image signals of B, G, R and Bk which have beenimage-processed in succession as a one image signal on a time basis.

Then, image forming controller 55 controlled by process controller 53controls image forming unit B as follows.

Namely, when the image signals are inputted in the laser writing unit20, a semiconductor laser (not shown) in the laser writing unit 20irradiates a laser beam. The laser beam hits polygon mirror 201 which isrotated by a driving motor (not shown) and is deviated after reflectionon the polygon mirror 201, and is projected, in a scanning manner, onthe peripheral surface of the photoreceptor belt 15 which is uniformlycharged by charging unit 19 in advance to a level of predeterminedcharges, after being led through fθ lens, cylindrical lens 204 and threemirrors 203, so that the laser beam may form an emission line on thesurface of the photoreceptor belt 15.

With regard to the direction making a right angle with the emissionline, which is called a sub-scanning direction, on the other hand, abelt index, (not shown) corresponding to the specific position on thephotoreceptor belt 15, is detected, or print commanding signals arereceived, and a main-scanning line with which modulation of thesemiconductor laser by means of image signals, is determined based onthe detection or commanding signals which serve as a standard. Afterscanning is started, with regard to the main-scanning direction, a laserbeam is detected by an index sensor (not shown), modulation of thesemiconductor laser by means of image signals for the color is startedbased on the standard of the detected signals, and the modulated laserbeam scans the surface of the photoreceptor belt 15. Therefore, owing toboth main-scanning by means of a laser beam and sub-scanning by means ofconveyance of the photoreceptor belt 15, a latent image for the firstcolor is formed on the surface of the photoreceptor belt 15 which isuniformly charged. This latent image is developed by developer 21Ycontaining yellow toners corresponding to the first color among pluraldeveloping means and thereby a yellow toner image is formed on thesurface of the photoreceptor belt 15. After that, the photoreceptor belt15, having thereon a yellow toner image, passes through the under sideof cleaning blade 231 that is kept away from the surface of thephotoreceptor belt 15, and then it enters a cycle of image forming forthe second color.

Similarly to image forming for the first color, mentioned above, thephotoreceptor belt 15, on which a yellow toner image is formed, ischarged by charging unit 19 again, and then, image signals for thesecond color, (magenta) read and image-processed by image reading unitA, are inputted into the laser writing unit 20. Thus, writing on thesurface of the photoreceptor belt 15 is conducted in the same manner asin the case of image signals for the aforementioned first color, and alatent image is formed. The latent image is developed by developer 21Mcontaining magenta toners as the second color. A magenta toner image isformed in the presence of the yellow toner image which has already beenformed.

In the same manner, a latent image formed by image signals for the thirdcolor is developed by developer 21C containing cyan toners, and a cyantoner image is formed. Further, a latent image formed by image signalsfor the fourth color is developed by developer 21Bk containing blacktoners, and a black toner image thus obtained is superimposed on thesurface of the photoreceptor belt 15 and thereby a color toner image isformed on the surface of the photoreceptor belt 15.

On each of these developers 21, DC or DC plus AC bias voltage isimpressed so that reversal development (jumping development) may beconducted, on a non-contact basis, on the photoreceptor belt 15 whosebase substance is grounded. Further, developers which are not requiredfor development are do not operate so that unnecessary toners are notsupplied to a latent image and toner images on the photoreceptor belt 15are not damaged.

Incidentally, any of mono-component developing agent and two-componentdeveloping agent may be used for the non-contact development. When amono-component developing agent is used, it is not necessary to providea toner hopper and a toner concentration detecting means, separatelyfrom the developing unit, and thereby, it is possible to make theapparatus small. From the viewpoint of stable development, however, adevelopment method employing a two-component developing agent is morepreferable.

As stated above, a color toner image, formed on the surface of thephotoreceptor belt 15, is transferred onto a transfer material which isfed from sheet cassette 27 by means of sheet feeding roller 28 and issynchronized with the aforesaid color toner image by means of timingroller 29. Transfer unit 22 is impressed with high voltage power sourcewhose polarity is opposite to that of toners, for transferring.

The transfer material on which color toner images have been transferredis separated by the photoreceptor belt 15 which changes its directionsharply (with a small radius of curvature) along driving roller 16, andis conveyed upward by conveyance belt 30. Incidentally, the conveyancebelt 30 is equipped with sucking means 301, and thereby the transfermaterial can be conveyed upwardly surely while being sucked. Then, thetransfer material, after toners thereon are melted and fixed by fixingroller 31, that is a fixing means, is ejected out of the apparatus mainbody 10 by sheet ejection roller 32.

On the other hand, the photoreceptor belt 15 from which color tonerimages have been transferred to a transfer material is further movedclockwise, and thereby toners staying on the photoreceptor belt 15 areremoved by cleaning means 23 whose cleaning blade 231 is inpressure-contact with the belt, for cleaning thereof. After cleaning iscompleted, the cleaning blade 231 is brought away from the photoreceptorbelt 15 to enter the following image forming process.

FIG. 3 shows a timing chart of a process relating to copying of a colordocument mentioned above. Incidentally, when each means is in operation,it is shown with a high level. Further, E_(Y1), E_(M1), E_(C1) andE_(B1) in FIG. 3 correspond to so-called standard patches written forstandard toner image forming. These patches are developed respectivelyby developers 21Y, 21M, 21C and 21Bk with developing timing of D_(Y),D_(M), D_(C) and D_(B), and toner images thus obtained are measured byreflection densitometer 33 for detection of development conditions. Dataobtained from the measurement are fed back to the adjusting means foreach of toner concentration, charging voltage, amount of exposure anddevelopment bias, for adjustment.

Further, each of R₁, R₂, R₃ and R₄ in FIG. 3 represents timing withwhich the image reading unit A reads a document placed on document table11, while V_(Y), V_(M), V_(C) and V_(B) represent respectively chargingtiming of development for yellow, magenta, cyan and black. E_(Y2),E_(M2), E_(C2) and E_(B2) respectively represent timing for writing withlaser writing unit 20 the image signals respectively for yellow,magenta, cyan and black read and image-processed respectively withtiming R₁, R₂, R₃ and R₄, and latent images formed through writingmentioned above are visualized respectively by developers 21Y, 21M, 21Cand 21Bk for yellow, magenta, cyan and black with timing D_(Y), D_(M),D_(C) and D_(B) respectively including standard patches and image areas.

Hereinafter, the process mentioned above is called a 1-round process (or1-page process) because one image of one color is formed while thephotoreceptor belt 15 makes one turn.

Copy of A-3 Size

The process for copy of A-3 size is mostly the same as that for copy ofA-4 size (1-round process) mentioned above. However, a reading opticalunit composed of carriage 12 and movable mirror 13 can not return to itsinitial position while the photoreceptor belt 15 whose circumferentiallength is marginal against A-3 size makes one turn, and therefore, imagereading unit A cannot synchronize with the photoreceptor belt 15.Therefore, the process for copy of A-3 size is one for overcoming theabove problems.

Similarly to image forming of the aforementioned 1-round process, thephotoreceptor belt 15 is Charged by charging unit 19 again, then imagesignals for the first color (yellow) read and image-processed by imagereading unit A and selected by selector 510 are inputted into theaforementioned laser writing unit 20 and then written on the surface ofthe photoreceptor belt 15 to form a latent image, similarly to the caseof the aforesaid 1-round process. The latent image is developed bydeveloper 21Y containing yellow toners as the first color. After that,the photoreceptor belt 15 having thereon a yellow toner image passesthrough the under side of cleaning blade 231 that is kept away from thesurface of the photoreceptor belt 15. Unlike the previous 1-roundprocess, the photoreceptor belt 15 is conveyed again. The reason behindthis is to cause a reading optical unit composed of carriage 12 andmovable mirror 13 to return to its initial position in the period of theadditional one rotation of the photoreceptor belt 15 because the readingoptical unit cannot return to its initial position during the firstrotation of the photoreceptor belt 15. The photoreceptor belt 15 passesagain through the under side of the cleaning blade 231 being kept awayfrom the surface of the photoreceptor belt 15, while having thereon ayellow toner image. After that, in the third rotation of thephotoreceptor belt 15, image forming for the second color of magenta isconducted synchronizing with the yellow toner image.

In the period of the third rotation of the photoreceptor belt 15, alatent image, formed with image signals for the second color, isdeveloped by developer 21M containing magenta toners and thereby amagenta toner image is formed. In the period of the fifth rotation ofthe photoreceptor belt 15, a latent image formed with image signals forthe third color is developed by developer 21C containing cyan toners andthereby a cyan toner image is formed, and in the period of the seventhrotation of the photoreceptor belt 15, a latent image formed with imagesignals for the fourth color is developed by developer 21Bk containingblack toners and the black toner image thus obtained is superimposed onthe surface of the photoreceptor belt. Thus a complete color toner imageis formed on the surface of the photoreceptor belt 15.

After a color toner image is formed on the photoreceptor belt 15,transferring and fixing of the image on the transfer material areconducted and the transfer material is ejected.

FIG. 4 shows a timing chart for the process related to the case forcopying the aforesaid document on the transfer material in A-3 size.Again, when each means is in operation, it is shown with a high level.Further, E_(Y1), E_(M1), E_(C1) and E_(B1) in FIG. 4 correspond toso-called standard patches written for standard toner image forming.These patches are developed respectively by developers 21Y, 21M, 21C and21Bk with developing timing of D_(Y), D_(M), D_(C) and D_(B), and tonerimages thus obtained are measured by reflection densitometer 33 fordetection of development conditions. Data obtained from the measurementare fed back to the adjusting means for each of toner concentration,charging voltage, amount of exposure and development bias, foradjustment.

Further, each of R₁, R₂, R₃ and R₄ in FIG. 4 represents timing withwhich the image reading unit A reads a document placed on document table11, while V_(Y), V_(M), V_(C) and V_(B) represent respectively chargingtiming for development for Yellow, magenta, cyan and black. E_(Y2),E_(M2), E_(C2) and E_(B2) respectively represent timing for writing withlaser writing unit 20 the image signals respectively for yellow,magenta, cyan and black, read and image-processed respectively withtiming R₁, R₂, R₃ and R₄, and latent images formed through writingmentioned above are visualized respectively by developers 21Y, 21M, 21Cand 21Bk for yellow, magenta, cyan and black with timing D_(Y), D_(M),D_(C) and D_(B) respectively including standard patches and image areas.

In FIG. 4, when making copies on a transfer material in A-3 size, imageforming is conducted within the first rotation of the photoreceptor belt15 synchronizing with a reading optical unit of image reading unit A,and in the second rotation of the photoreceptor belt 15, thescan-reading optical unit is returned to its initial position. Afterthat, the photoreceptor belt 15 and the image reading unit A aresynchronized for image forming in each odd-numbered rotation of thephotoreceptor belt 15, and the scan-reading optical unit is returned toits initial position in each even-numbered rotation of the photoreceptorbelt 15. Namely, in copying for A-3 size, one cycle of reading an imageon a document and forming of an image is conducted for every tworotations of the photoreceptor belt 15.

Hereinafter, the process mentioned above is called a 2-round processbecause one image of one color is formed while the photoreceptor belt 15makes two turns.

Print Mode

Next, a print mode in which an image forming unit of the presentapparatus forms an image on a transfer material using image data from aperipheral device such as a computer or a word processor, namely aprocess wherein image forming unit B of the aforesaid color imageforming apparatus is used for forming a color image on a transfermaterial, will be explained, referring to FIGS. 1, 2 and 5.

Process control means 53 selects a process based on image sizeinformation among image signals for image forming stored in peripheralinput device 52. Namely, the process control means 53 selects either theaforesaid 1-round (page) process or a 2-page process, which will beexplained later, depending on an image size of one page to be formed,and controls image forming unit B based on the process selected.

In the print mode, image signals are outputted from a page memory ofperipheral output device 52 directly to laser writing unit 20 andtherefore, it is not necessary to control image reading unit A, which isdifferent from the aforementioned copy mode. Therefore, the followingtwo cases are exemplified separately, because it is possible to form animage to the maximum size capable of being formed on the photoreceptorbelt 15. Since the maximum image size is A-3 in the present example,size of an image to be formed is separated into A-4 or smaller than thatand B-4 or larger than that. Since no image reading unit A is used, evenwhen an image size is the maximum of A-3, it does not need to bediscriminated in particular, which is different from the aforesaid copymode.

An example wherein image forming is conducted both on an A-4 sizetransfer material and on an A-3 size transfer material will be explainedas follows.

Print of A-4 Size

Based upon image size information (information that an image size isA-4) coming from peripheral output device 52, the process control means53 outputs print signals of 2-page process to peripheral output device52 and to image forming unit controller 55. Based on signals from theprocess control means 53, the peripheral output device 52 and imageforming unit 55 conduct image forming in the 2-page process as followsaccording to the print signals.

The peripheral output device 52 outputs respective image signals ofyellow, magenta, cyan and black for forming images to laser writing unit20 twice each, synchronizing with image forming unit B.

On the other hand, image forming controller 55, controlled by processcontroller 53, controls the image forming unit B.

Namely, as in the case of image forming in the copy mode, thephotoreceptor belt 15 on which a yellow toner image has been formed ischarged by charging unit 19 again, and then, image signals for the firstcolor (yellow) outputted from peripheral device 52 are inputted into thelaser writing unit 20 and thereby writing on the surface of thephotoreceptor belt 15 is conducted to form a latent image for the firstimage page.

Further, when the latent image for the first image page has been formed,the photoreceptor belt 15 still has an excessive length for forming alatent image for the second image page. Therefore, image signals foryellow are outputted immediately to the laser writing unit 20 andthereby a latent image for the second image page is formed in the samemanner as in the foregoing.

Accordingly, two latent images for two image pages corresponding toimage signals for yellow are formed on the photoreceptor belt 15.

These latent images are developed by developer 21Y containing yellowtoners corresponding to the first color among developing means, thus, ayellow toner image is formed on the surface of the photoreceptor belt15. After that, the photoreceptor belt 15, while keeping thereon theyellow toner image, passes through the under side of cleaning blade 231which is kept away from the surface of the photoreceptor belt 15, andenters image forming for the second color.

A latent image for the second image page formed by image signals for thesecond color is developed by developer 21M containing magenta toners inthe same way as in the foregoing to form a magenta toner image, a latentimage for the second image page formed by image signals for the thirdcolor is developed by developer 21C containing cyan toners to form acyan toner image, and further, a latent image for the second image pageformed by image signals for the fourth color is developed by developer21Bk containing black toners to superimpose a black toner image on thesurface of the photoreceptor belt 15. Thus, a complete color toner imageis formed on the surface of the photoreceptor belt 15.

The color toner image for the second image page formed on the surface ofthe photoreceptor belt 15, as explained above, is transferred onto atransfer material which is fed out from sheet feeding cassette 27 bysheet feeding roller 28 and is synchronized by timing roller 29 with acolor toner image for the first image page. After that, the color tonerimage for the second image page is transferred onto a transfer materialsynchronized with the color toner image for the second image page.

After the foregoing, the toner image on the photoreceptor belt 15 istransferred onto a transfer material and fixed thereon, and the transfermaterial is ejected out of an apparatus in the same process as in theaforementioned copy mode.

FIG. 5 shows a timing chart of a process related to continuous printingof color images each being of the aforementioned A-4 size. Incidentally,when each means is in operation, it is shown with a high level. Further,E_(Y1), E_(M1), E_(C1) and E_(B1) in FIG. 5 correspond to so-calledstandard patches written for standard toner image forming. These patchesare developed respectively by developers 21Y, 21M, 21C and 21Bk withdeveloping timing of D_(Y1), D_(M1), D_(C1) and D_(B1), and dataobtained from the development are fed back to the adjusting means foreach of toner concentration, charging voltage, amount of exposure anddevelopment bias, for adjustment.

R₁, R₂, R₃, and R₄ in FIG. 5 show output of image signals for yellow,magenta, cyan and black for the first image page outputted fromperipheral device 52, R₁ ', R₂ ', R₃ ' and R₄ ' show image signals ofyellow, magenta, cyan and black for the second image page following theimage signal output for the first image page outputted from peripheraldevice 52. V_(Y), V_(M), V_(C) and V_(B) respectively show chargingtiming of development for each of yellow, magenta, cyan and blacktoners, and E_(Y2), E_(M2), E_(C2) and E_(B2) as well as E_(Y2) ',E_(M2) ', E_(C2) ' and E_(B2) ' respectively show timing with whichimage signals for yellow, magenta, cyan and black are written by laserwriting unit 20 with timing of R₁, R₂, R₃ and R₄ as well as R₁ ', R₂ ',R₃ ' and R₄ ', and latent images formed by writing by a laser arevisualized by developers 21Y, 21M, 21C and 21Bk respectively for yellow,magenta, cyan and black with timing D_(Y), D_(M), D_(C) and D_(B)including standard patches and image forming areas.

The process mentioned above, is called a 2-page process hereinafter,because two image pages for one color are formed during a period of onerotation of the photoreceptor belt 15.

Incidentally, in the case of the 2-page process, it is preferable that acertain clearance is given between the first image page and the secondimage page. The reason behind this is for the easy synchronizationbetween a transfer material and a toner image on an image formingmember. For this purpose, image data for the second image page may bedelayed by a process controller to be outputted to image forming unit B.

Further, although there has been shown an example wherein two colorimages of the same image data are formed in the present example, theinvention is not limited to this, and image data for the first imagepage may naturally be different from those for the second image page. Inthis case, process controller 53 controls so that image signalsoutputted from peripheral device 52 may be outputted to laser writingunit 20 in the sequence of image signals of yellow for the first imagepage, image signals of yellow for the second image page, image signalsof magenta for the first image page, . . . , image signals of black forthe second image page.

Print of A-3 Size

Based on an image size information (information that the image size isA-3) from input controlling unit 54, the process controller 53 outputsprint signals for the first image page process to peripheral device 52and to image forming controller 55. Then, based on signals from theprocess controller 53, the peripheral device 52 and image forming unit55 form an image through 1-page process based on the aforementionedprint signals.

The 1-page process is the same as the 1-round process in theaforementioned copy mode. However, in the 1-page process in the copymode, image signals obtained from image reading unit A are inputted inlaser writing unit 20, while image signals stored in peripheral device52 are inputted in laser writing unit 20 in the 1-page process in theprint mode. With regard to latent image forming, a latent image isformed according to image signals of the peripheral device 52.

Further, a timing chart for the 1-page process in the print mode is thesame as that shown in FIG. 3 for the copy mode, although the image widthis greater. However, R₁, R₂, R₃ and R₄ showing timing for reading adocument on document table 11 with image reading unit A are thoseshowing image signals for yellow, magenta, cyan and black outputted fromthe peripheral device 52, in which the period of time for the imagereading optical unit to return is not needed.

Selection of 1-round process (1-page process), 2-round process or 2-pageprocess depending on size for image forming in the copy mode or theprint mode on an image forming apparatus mentioned in detail above isconducted by the process controller 53. A flow chart is shown in FIG. 6.

When a start button for image forming is pressed, a copy mode or a printmode is judged by the process controller 53. This judgment is made bythe signal from a button that is provided on an operation panel and isfor changeover for copy mode/print mode in which the selection is madein advance.

In case of a copy mode, the process controller 53 judges whether thesize of an image to be formed is A-3 or not from information includingsetting made by an operator, size of paper to be used and image sizeobtained from size detection by means of image reading unit A.

In case of the size other than A-3 size, the process controller 53 sendscommands to image forming controller 55 so that an image may be formedthrough the 1-round process, then, the image forming controller 55conducts image forming through the aforesaid 1-round process based onthe aforementioned commands, by causing the image reading unit A andimage forming unit B to be synchronized each other.

In case of the size of A-3, on the contrary, the process controller 53sends commands to the image forming controller 55 so that an image maybe formed through the 2-round process, and the image forming controller55 causes the image forming unit B to conduct image forming through the2-round process, based on the aforementioned commands.

In the case of a print mode, on the other hand, the process controller53 judges whether or not the size of an image to be formed is not largerthan A-4, from image size information outputted from the peripheraldevice 52.

When the size is not larger than A-4 size, the process controller 53sends commands to the image forming controller 55 so that an image maybe formed through the 2-page process, and the image forming controller55 causes the image forming unit B to conduct image forming through the2-page process, based on the commands mentioned above.

When the size is not larger than A-4 size, on the contrary, the processcontroller 53 sends commands to the image forming controller 55 so thatan image may be formed through the 1-page process, as in the case of the1-round process mentioned above, and the image forming controller 55causes the image forming unit B to conduct image forming through the1-page process, based on the commands mentioned above.

In the 1-round process or the 2-round process in the copy mode, or the1-page process or the 2-page process in the print mode, selected asexplained above, image forming is conducted through each process statedabove.

Incidentally, although image forming is conducted through the aforesaid1-round process under the conditions of the copy mode and transfermaterial size of A-3 size or less, the circumferential length of thephotoreceptor belt 15 is determined to be 420+α for the maximum sheetsize of A-3 in the present example. Therefore, when image forming isconducted on a transfer material whose size is smaller than one half ofA-3, that is, B-5 size or smaller, the image reading unit A can read adocument twice during a period of one rotation of the photoreceptor belt15, and an image reading optical unit further can return. Accordingly,image forming can be conducted also in the 2-page process.

Further, in the present example, the maximum sheet size is A-3, and whenimage forming is conducted on a transfer material of A-3 size in a copymode, one image page is formed during two rotations, and when imageforming is conducted on an A-4 size transfer material in a print mode,two image pages are formed simultaneously. However, the invention is notlimited to this. Corresponding to the maximum size for image forming,the size changes to the transfer material of that size.

Selection of either a copy mode or a print mode may be made either by achangeover button provided on the apparatus main body 10 or by a copybutton as well as a print button (both not shown). It may also beapplicable that a print mode is selected only for print signals from aperipheral output device 52 and when a button provided on an operationpanel (not shown) of the apparatus main body 10 is pressed, imageforming is conducted in a copy mode.

FIG. 7 shows a color image forming apparatus of a transfer drum type asanother example in which the present invention may be applied.Incidentally, the numbers used in FIG. 6 which are the same as thoseused in FIG. 1 represent those having the same structure or function asin the aforesaid color image forming apparatus.

In the color image forming apparatus 10 of a transfer drum type,transfer drum 40 is arranged so that it may touch the transfer portionon the photoreceptor belt 15, and the transfer drum 40 rotatescounterclockwise, synchronizing with the photoreceptor belt 15. Atransfer material is wound around the transfer drum 40, and a tonerimage of each color formed on the transfer belt 15 is transferred ontothe transfer material in a manner such that toner images each having itsown different color are superposed on the transfer material. After theyare superimposed on the transfer material, the transfer material isseparated from the transfer drum and ejected out of the apparatus mainbody 10.

Incidentally, in the color image forming apparatus in the presentexample, the circumferential length of 420+α mm is not for thephotoreceptor belt 15 but for the transfer drum, which is different fromthe color image forming apparatus in the aforementioned example.

Charging unit 401 functions so that a transfer material may be attractedelectrostatically to the transfer drum and winding member 402 winds thetransfer material round the transfer drum 40 mechanically are providedaround the transfer drum 40. The winding member 402 has a roller on itstip and it touches only when the transfer material is wound around thetransfer drum 40 first. Further, on the peripheral surface of thetransfer drum 40, there are provided grippers 403 and 404 which hold theleading edge of the transfer material conveyed synchronously. Transferunit 405 transfers toner images on the photoreceptor belt 15electrostatically onto the transfer material. Both separationneutralizing electrode 406 and separation claw 407 separate the transfermaterial after transferring from the transfer drum 40. Cleaner 408capable of touching the transfer drum and leaving therefrom removesresidual toners staying on the transfer drum 40 after the transfermaterial has been separated.

A transfer material fed out of cassette 27 synchronously advances totransfer drum 40 charged by charging unit 401, and is wound around thetransfer drum by winding member 402. The leading edge of the transfermaterial is held by gripper 403 or by grippers 403 and 404 and moves toa transfer portion where a yellow toner image formed on thephotoreceptor belt 15 is transferred onto the transfer material bytransfer unit 405. The transfer drum 40, on which the first transferringis completed, continues to be cleaned by cleaner 508 for transferring ofthe next toner image. Namely, a magenta toner image is transferred inthe second rotation of the transfer drum, a cyan toner image istransferred in the third rotation and a black toner image is transferredin the fourth rotation, so that all images are superimposed. After tonerimages for the respective four colors have been transferred, thetransfer material is neutralized by separation neutralizing electrode406 and is separated by separation claw 407 after the leading edge ofthe transfer material is released from being held. Then, the transfermaterial is conveyed to fixing roller 31.

The present example is mostly the same as the aforementioned example.Namely, it is controlled based on a control block diagram related toprocess controller 53 and image processing means 50, shown in FIG. 2.Further, in the same manner as in the aforementioned example, theprocess controller 53 selects 1-round process or 2-round process in acopy mode, or 1-page process or 2-page process in a print mode, based onthe flow chart shown in FIG. 6. Further, toner images are formed on thephotoreceptor belt 15, based on time charts for 1-round (page) process,2-round process and 2-page process shown in FIGS. 3 and 4. In this case,however, a developed toner image is transferred onto a transfer materialfor each color without being superimposed with images of other differentcolors.

Incidentally, in the aforementioned example, a color toner image istransferred onto a transfer material when the transfer material passesthrough a transfer portion after being fed out of sheet cassette 27. Inthe present example, however, a transfer material fed out of sheetcassette 27 is held on its leading edge by gripper 403 in a 1-round(page) process, and is held on its leading edge by grippers 403 and 404in a 2-page process. Further, in the 2-page process, the leading edge ofthe second image page is determined by gripper 404. Therefore, thedistance between the first image page and the second image page, whoselatent images are formed on the photoreceptor belt 15 by laser writingunit 20, is the same as that between gripper 403 and gripper 404 on thecircumferential surface of the transfer drum 40.

As stated above in detail, the present invention provides a color imageforming apparatus wherein depending upon an image size, image formingmay be conducted through either a 1-round process or a 2-round processin a copy mode, while, in a print mode, image forming may be conductedbased on either a 1-page process or a 2-page process, whichever isselected.

As a result, when image size is small, a color image forming apparatusof the invention has advantages that a wasteful portion on an imageforming member may be eliminated and operation efficiency may beenhanced by increasing the number of times for image forming in colorimage forming operation wherein image forming for a smaller image sizeis more complicated than that for a larger image size. Further, it ispossible to make the circumferential length of an image forming membershorter and thereby to make the apparatus small in size and light inweight. Furthermore, the invention offers an advantage that efficientimage forming can be conducted with optimum number of times for imageforming in any mode of image forming.

What is claimed is:
 1. A color image forming apparatus for formingimages corresponding to original image data, from at least one documenton recording sheets, comprising:means for scanning the document so thatsaid original image data is obtained; means for receiving said originalimage data from peripheral image data generating apparatus; means forselecting one of a scanning mode, wherein said scanning means obtainssaid original image data, and a printing mode, wherein said receivingmeans receives said original image data; means for obtaining a pluralityof color component images corresponding to said original image data;means for forming a plurality of color toner layers on an image holdingmeans as the image, wherein each of said color toner layers correspondsto one of said color component images, said image holding means, havinga rotatable endless surface, for holding said color toner layersthereon; and means for transferring said color toner layers onto saidrecording sheet, wherein, when said scanning mode is selected, saidimage holding means rotates a predetermined number of times according tothe size of said image when forming one of said color toner layers onsaid image holding means, and, when said printing mode is selected, saidforming means forms each of said color toner layers for at least onedocument according to the size of said image while said image holdingmeans rotates once.
 2. The apparatus of claim 1, wherein, when saidscanning mode is selected, said image holding means rotates twice whenthe size of said image is the maximum size that can be formed by theapparatus when forming one of said color toner layers on said imageholding means.
 3. The apparatus of claim 1, wherein, when said scanningmode is selected, said image holding means rotates when the size of saidimage is less than the maximum size that can be formed by the apparatuswhen forming one of said color toner layers on said image means.
 4. Theapparatus of claim 1, wherein, when said printing mode is selected, saidforming means forms each of said color toner layers for two documentswhen the size of said image is not more than one half of the maximumsize that can be formed by the apparatus, while said image holding meansrotates once.
 5. The apparatus of claim 1, wherein, when said printingmode is selected, said forming means forms each of said color tonerlayers for one document when the size of said image is more than a halfof the maximum size that can be formed by the apparatus, while saidimage holding means rotates once.
 6. A color image forming apparatus forforming an image corresponding to original image data from at least onedocument on at least one recording sheet, comprising;means for scanningthe document so that said original image data is obtained; means forreceiving said original image data from a peripheral image datagenerating apparatus; means for selecting one of a scanning mode,wherein said scanning means obtains said original image data, and aprinting mode, wherein said receiving means obtains said original imagedata; means for obtaining a plurality of color component imagescorresponding said original image data; means for forming a plurality ofcolor toner layers on an image holding means, wherein each of said colortoner layers corresponds to one of said color component images, saidimage holding means, having a rotatable endless surface, for holdingsaid color toner layers thereon; and means for transferring said colortoner layers onto said recording sheet, wherein, when said scanning modeis selected, said image holding means rotates for forming each of saidcolor toner layers on said image holding means so that a color tonerimage comprising said plurality of color toner layers is formed on saidrotatable endless surface, and, when said printing mode is selected,said forming means forms each of said color toner layers for at leastone of said documents while said image holding means rotates once, sothat a color toner image comprising said plurality of color toner layersis formed on said rotatable endless surface.
 7. The apparatus of claim6, wherein, when said printing mode is selected, said forming meansforms each of said color toner layers of two documents when the size ofsaid image is not more than one half of the maximum size that can beformed by the apparatus, while said image holding means rotates.
 8. Theapparatus of claim 6, wherein, when said printing mode is selected, saidforming means forms each of said color toner layers for one documentwhen the size of said image is more than one half of the maximum sizethat can be formed by the apparatus, while said image holding meansrotates once.